This invention relates to the synthesis of an alkali monofluorophosphate, and more particularly to a process of preparing sodium or potassium monofluorophosphate by reaction of sodium or potassium pyrophosphate, or disodium or dipotassium hydrogenphosphate, with hydrogen fluoride.
In the present application, the term "alkali monofluorophosphate" means a compound expressed by the formula M.sub.2 PO.sub.3 F, where M represents Na or K, and sometimes will be abbreviated to "MFP".
According to a method described in "Ind. Eng. Chem.", Vol. 43, 246-248 (1951), MFP is prepared by neutralizing monofluorophosphoric acid H.sub.2 PO.sub.3 F, which is formed by reaction between diphosphorus pentoxide and anhydrous hydrogen fluoride, by using an alkali metal salt. However, industrial applications of this method require the provision of high-grade and complicated apparatus and a great degree of skill in operation because of the use of highly toxic and corrosive materials and, hence, will suffer from high production costs.
In a different method, U.S. Pat. No. 2,481,807 discloses the preparation of MFP through melting of a mixture of a metaphosphate and an alkali fluoride. In this method, it is permissible to replace the metaphosphate by either a dihydrogenphosphate in the form of MH.sub.2 PO.sub.4 or a dihydrogenpyrophosphate in the form of M.sub.2 H.sub.2 P.sub.2 O.sub.7 which gives a metaphosphate at the temperature of the melting process. From an industrial viewpoint, this method is disadvantageous in that the reaction system must be heated to a temperature as high as 650.degree.-700.degree. C. and the melt formed in this process exerts errosive action on the reaction vessel with the inevitable result that considerable amounts of impurities intrude into the final product.